As computer systems have evolved so has the availability and configuration of data storage devices, such as magnetic or optical disks. For example, these storage devices can be connected to the computer system via a bus, or they can be connected to the computer system via a wired or wireless network. In addition, the storage devices can be separate or co-located in a single cabinet.
A storage volume is a software abstraction of the underlying storage devices and is commonly the smallest self-contained unit of storage exposed by an operating system and administered by a file system. Storage volumes abstract the physical topology of the storage devices and may be a fraction of a disk, a whole disk or even multiple disks that are bound into a contiguous range of logical blocks.
Volumes are constructed from one or more extents, with each extent being a contiguous storage address spaces presented by the underlying storage device. An extent is typically characterized by the size of the address space and a starting offset for the address space from a base of the media. Volume mapping is the process of mapping the contiguous address space presented by the volume onto the usually non-contiguous storage address spaces of the underlying extents. Volume mappings are either implemented on a specialized hardware controller, referred to as a hardware volume provider, or in software by a software volume provider.
Volume mappings may be used to increase the fault tolerance, performance, or capacity characteristics of the underlying storage devices. For example, a technique for improving fault tolerance, known as mirroring or plexing a disk, uses multiple disks. When data is written to one disk the data is also written to a second disk; thus the second disk is a “mirror image” of the first disk. If one disk should fail the other disk is still available for use and has an exact copy of the information on the first disk.
In addition RAID numbers are often used to identify storage volume mappings. A RAID, or Redundant Array of Independent Disks, provides the ability to lose an extent without losing volume data. Access to the volume may be slower or more costly, but is not interrupted by the failure of the underlying extent. RAID1 implements mirroring. RAID3 and above all implement some sort of stripe with parity scheme; the different number indicates the arrangement of the data and check-data (or parity) extents. Striping is a mechanism where data for a file or file system is distributed among several different disks.
Volume providers commonly group logical volumes into what are known as “diskpacks” in order to simplify volume management. The diskpack then is a collection of logical volumes and the underlying disks. Diskpacks provide transitive closure for the volumes contained in the disk pack and may provide group sanity checking to ensure volume configuration correctness.
Occasionally, it is necessary to “migrate” a volume from one diskpack to another. An example of a situation in which migration is required occurs when data is being partitioned among servers. Other examples include the replicating the data in order to perform a parallel analysis (e.g., data mining), or archiving the data.
Therefore, there is a need in the art for a system to perform volume migration. The system should preserve transitive closure of the migrated volumes, that is, the migration, or export, should preserve relationships that exist both within a volume, and between volumes. In addition, the system should automatically determine the disks or other components of the storage system that must be included in order to successfully migrate the volume. Finally, the system should implement a flexible unit of transfer for migrating volumes between diskpacks and between computer systems.